This invention relates generally to liquid crystal display devices and particularly to a thin film transistor (TFT) and conductor crossover structure for such devices.
Liquid crystal display devices include an array of liquid crystal elements (pixels) arranged in a matrix of rows and columns. In active matrix display devices, each pixel is associated with a solid state switching device, such as a thin film transistor (TFT). As is known to those skilled in the art, a liquid crystal element is composed of a liquid crystal material arranged between two electrodes. The light transmission capability of the liquid crystal material is changed by voltage biasing the electrodes to change the orientation of the molecules within the liquid crystal material. The liquid crystal elements are arranged in rows and columns, and individual elements are addressed by simultaneously voltage biasing a drive line and a data line which are individually coupled to the electrodes of the elements. The biasing voltage to one of the electrodes of the liquid crystal element is applied through a solid state switching device (TFT) to enable the liquid crystal elements to store a charge for the time required to produce a visual display. Accordingly, the fabrication of a liquid crystal display device includes the fabrication of the TFT's and the connection of the TFT's to the control electrodes of the liquid crystal elements.
Typically in the prior art, in the fabrication of the TFT's for liquid crystal displays various photographic masks are employed whereby different portions of the TFT's, and the liquid crystal electrodes, are formed at various steps in the process. Typically, the number of masks used in fabricating the TFT's is four. However, as is known to those in the art, the yield of devices fabricated using multiple masks is inversely proportional to the number of masks used. Accordingly, it is advantageous to decrease the number of masks used in the formation of the TFT's. In keeping with this goal, it is possible to fabricate the TFT's for liquid crystal display devices utilizing only two masks. The two mask process is advantageous when the drive circuitry used to energize the drive and data lines is fabricated separately from the solid state switching devices of the liquid crystal display. However, it should be appreciated that a substantial cost savings can be realized by simultaneously fabricating the TFT's and the drive circuitry onto the substrate which supports the liquid crystal matrix. The two mask processing has several disadvantages when the drive circuitry is to be so fabricated. Firstly, a parasitic transistor is formed and connects the pixel transistors to the data line, this parasitic transistor can deleteriously affect the on/off characteristics of adjacent TFT's or can cause crosstalk between adjacent data lines. Another disadvantage is the fact that the data lines have a high resistance. Another disadvantage is the lack of an electrical connection between the gate of the TFT and the source or drain lines.
For these reasons there is a need for a method for fabricating thin film transistors on the substrate of a liquid crystal display device which eliminates the above stated disadvantages. The present invention fulfills these needs by the provision of a three mask process which permits the interconnection of the gate and source/drain regions of various transistors. The invention also provides a crossover structure which enables the drive circuitry to be fabricated on the same substrate as the display device. Another advantage is the provision of a dielectric stringer which inhibits the formation of a parasitic resistor between the gate and channel regions of the TFT's.